The fabrication of pressed board by the compaction of a mat of wood chips, fibers, or dust or other cellulosic particles, with or without an extrinsic thermally-activated binder, can make use of cycling or continuous presses as will be described in greater detail below.
The presses can have press plates or platens which can be composed of steel and which can have mutually parallel steam-supply channels which can communicate with steam nozzle bores or orifices arrayed along each channel and opening at a surface of the platen turned toward the mat. The channels may communicate a steam distribution manifold.
Between the mouths of the orifices or nozzles, and the mats, screens or sieves of steel wire or porous ceramic may be provided to improve the steam distribution.
As noted, the presses may be cycling presses in which, in each cycle, the mat is charged onto a press platen when the press is open and the press is then closed to compact the mat between two platens. In continuously operated presses, the mat is entrained into the press between a pair of press belts which can be permeable so that the steam from the press platen can penetrate into the particle mass forming the mat. Usually both press platens or press plates are provided with the channels and orifices for feeding steam to the mat although, for the purposes of the present application, only a single such plate or platen may be described, it being understood that the opposite press plate or platen may likewise be similarly equipped with the steam channels and orifices.
The steam nozzle bores frequently are simply referred to as steam nozzles or orifices.
The mat comprises a particulate material which, as noted, can be sawdust or similar particles, wood chips or fibers, e.g. of wood or cellulose, and a binder. This binder may be natural resins present in the wood and/or thermally-activatable binders added separately to the particles and thus incorporated in the mat when the latter is shaped.
The steam serves to provide the heat required for the reaction of the binder in the mat which is generally under compression between the press plates or platens. The quantity of steam required is thermodynamically determined. It must supply the thermal energy required for the hardening process to the extent that the thermal energy is not supplied by other heating of the press platens.
In practice it has been found to be of considerable importance to provide a uniform distribution of the steam within the mat so that the latter is uniformly heated. The steam channels are thus uniformly distributed and are generally equidistant from one another and the bores are generally uniformly distributed over the area of the press platen.
However, in spite of the efforts to maintain a uniform feed of the steam of the pressed board or mat within the press, it has been found that the flow of steam is nonuniform in practice. For example, the flow velocity falls off with distance along the flow channel and the transition between the nozzle and feed channel requires a direction change so there may be local counterflow which may interfere with uniform distribution of the steam. With the length of the flow passage, moreover, the temperature also tends to fall off. As a consequence, the uniformity of the distribution of the steam to the mat to be pressed requires improvement. Indeed, one of the approaches to such improvement has been the use of screens in the aforementioned manner between the press platen and the mat.